1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to turbine airfoils with a serpentine flow cooling circuit.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine produces mechanical energy from the burning of hydrocarbons such as natural gas and oil. In a gas turbine engine, such as an industrial gas turbine engine (IGT), a compressor provides compressed air to a combustor, where the fuel is burned and an extremely hot gas flow produced. The hot gas flow is passed I through a turbine of multiple stages in order to convert the energy from the hot gas flow into mechanical energy that drives the turbine shaft. In order to increase the efficiency of the engine, the hot gas flow into the turbine is increased. The highest temperature usable is dependent upon the material properties of the turbine. The first stage stator vanes and rotor blades in the turbine are exposed to the hottest temperature. Thus, the maximum temperature is limited to the maximum temperature limits for these parts.
One method of allowing for even higher temperatures in the turbine is to provide cooling air for the vanes and blades in the turbine. complex cooling circuits have been proposed to provide for the maximum amount of airfoil cooling while using the minimum amount of cooling air. Since the cooling air used within the airfoil passages is generally diverted from the compressor (bleed off air), minimizing the amount of bleed off air required for cooling also will increase the efficiency of the engine. Hot spots on the airfoils are also a problem that must be dealt with. Because of the complex cooling circuits, some parts of the airfoil may be over-cooled while another part may be under-cooled.
Prior art airfoil cooling include the use of a triple pass serpentine flow cooling circuit as shown in FIG. 1. This includes a forward flowing triple pass and an aft flowing flow circuit. The forward flowing flow circuit normally is designed in conjunction with leading backside impingement plus showerhead and pressure side and suction side film discharge cooling holes. The aft flowing serpentine flow circuit is designed in conjunction with airfoil trailing edge discharge cooling holes. This type of cooling flow circuit is called a dual triple pass serpentine “warm bridge” cooling concept. The forward flowing serpentine circuit includes a first leg 11 having an upward flow direction, a second leg 12 with a downward flow direction, and a third leg 13 with an upward flow direction. A leading edge supply channel 14 with showerhead cooling holes 15 discharges cooling air, and metering holes 16 supply cooling air from the third leg 13 to the supply channel 14. The aft flowing serpentine circuit includes a first leg 21 with an upward flow direction, a second leg 22 with a downward flow direction, and a third leg 23 with an upward flow direction, and exit cooling holes 24 connected to the third leg 23.
Another prior art cooling flow circuit is shown in FIG. 2. This is a dual triple pass serpentine flow circuit for a high operating gas temperature and is referred to as the “cold bridge” cooling concept. In this particular design, the leading edge airfoil is cooled with a self-contained flow circuit. The airfoil mid-chord section is cooled with a triple pass serpentine flow circuit. However, the aft flow circuit is flowing forward instead of aft ward like in the warm bridge design of FIG. 1. The aft flowing serpentine flow circuit is designed in conjunction with airfoil trailing edge discharge cooling holes. The mid-chord serpentine flow circuit includes a first leg 31 with an upward flow direction, a second leg 32 with a downward flow direction, and a third leg 33 with an upward flow direction. The self-contained leading edge cooling circuit includes a supply channel 35, a metering hole 38, a leading edge channel 36, and a showerhead arrangement of cooling holes 37. The aft flow serpentine circuit includes a first leg 41 with an upward flow direction, a second leg 42 with a downward flow direction, and a third leg 43 with an upward flow direction. Trailing edge exit holes are connected to the first leg 41.
In both the warm bridge and the cold bridge designs of the prior art above, the internal cavities are constructed with internal ribs connecting the airfoil pressure and suction walls. In most of the cases, the internal cooling cavities are at low aspect ration which is subject to high rotational effects on the cooling side heat transfer coefficient. The low aspect ration cavity yields a very low internal cooling side convective area ratio to the airfoil hot gas external surface.
An object of the present invention is to provide for an airfoil serpentine cooling circuit which optimizes the airfoil mass average sectional metal temperature to improve airfoil creep capability for a blade cooling design.